The present invention relates to an optical disc and more particularly to an optical disc having a high recording density by using a sampled servo tracking method.
As a recording format for the optical disc, a sampled servo tracking method is known. The optical disc using the sampled servo tracking method is provided with a preformat (pits) on a recording film comprising servo areas (fields) at 1376 points on a track. By sampling the servo area, tracking error and clock for recording and reproducing are generated.
FIG. 7 shows a conventional optical disc DK dependent on a recording format of the sampled servo tracking method. The optical disc DK has a program area PA in which a spiral track is formed from the inner portion to the outer periphery of the disc to provide a plurality of tracks arranged in the radial direction of the disc. One track is divided into 32 sectors. Each sector has 43 segments. A first segment #0 of the 43 segments has a preformat comprising a synchronizing signal Ssync of 1 byte for synchronizing at each sector unit and an address S.sub.ADR of 1 byte for addressing the sector unit. The preformat is formed in the process of mastering of the optical disc. Each of the other segments #1 to #42 has 18 bytes comprising a servo area Fs of 2 bytes and a data area FD of 16 bytes.
FIG. 8 shows the servo area Fs of 2 bytes comprising a first servo byte #1 and a second servo byte #2. The first servo byte #1 has a preformat comprising a first wobbled tracking pit Pw1 formed at a third bit and a second wobbled tracking pit Pw2 formed at an eighth bit in a first 16-track (A). The first tracking pit Pw1 is inwardly deflected from a track center Tc in the radial direction by 1/4 track pitch and the second tracking pit Pw2 is outwardly deflected from the track center Tc by 1/4 track pitch. A tracking error is detected by the difference between the reflected lights at first and second tracking pits Pw1 and Pw2.
The second servo byte #2 has a clock pit CP for synchronization. Which is preformatted at a twelfth bit. Between the second tracking pit Pw2 and the clock pit CP, a mirror surface having a distance of 19-channel clock length is provided. The 19-channel clock is counted for synchronizing each segment. Furthermore, a focus error is detected in the synchronization detecting period.
At a next 16-track (B), a first tracking pit Pw1 is provided at a fourth bit. Since the position of the first tracking pit Pw1 is changed between the 3 bit and 4 bit at every 16-track, the number of tracks during search is detected with accuracy.
When the servo area Fs is irradiated with a laser beam, a detected signal including a tracking signal ST1 (ST1A and ST1B) and a synchronizing signal Ssync is generated as shown in FIG. 8.
Thus, in the sampled servo tracking method, various information signals such as the tracking error signal and focus error signal are derived from the wobbled tracking pits Pw1, Pw2, and clock pits which are formed beforehand as prepits on the optical disc. In order to obtain the information, a laser beam is radiated on pits.
FIG. 9a shows signal pits PT in section formed on the disc DK. When information recorded on the disc is read, the pits are irradiated with the laser beam. The intensity of the reflected light is low at the pit PT. On the disc, a mirror surface is formed between the pits, so that the laser beam is entirely reflected at the mirror surface. Thus, the intensity of the light becomes high.
Consequently, it is necessary to correctly read pits in order to read servo information with accuracy. Conventionally, a track pitch Tp is determined larger than a diameter BL of a beam spot of the laser beam (about 1.6 .mu.m).
As shown in FIG. 9a, the width of the land is L, the width of the pit is 3L, the diameter BL of the beam spot is 3L, and the track pitch Tp=4L.
In order to increase the recording density of the disc, it is considered to reduce the track pitch Tp by one half, about 0.8 .mu.m, as shown in FIGS. 9b and 9c. In FIG. 9c, the axis of the laser beam is on the track center, it is called an on-track state. In FIG. 9b, the axis of the laser beam is deflected from the track center, it is called an off-track state. In those states, the difference between the intensity of the reflected light in the on-track state and the intensity of the reflected light in the off-track state is small so that the tracking servo is not accurately operated. Therefore it is difficult to reduce the track pitch.
In order to solve the problem, the applicant of the present invention proposed a tracking pit recording method for a CAV optical disc disclosed in Japanese Patent Application No. 3-64978. FIG. 10 shows a recording format of the CAV optical disc. The disc has tracks 2k, 2k+1, 2k+2, 2k+3 and 2k+4. On the track 2k, tracking pits Pw(2k-1) and Pw(2k) are provided opposite to each other with respect to a track center and are separated by a pitch L. A synchronizing pit PSYNC and a discriminating pit P.sub.DET are formed on the track center. The tracking pit Pw(2k-1) corresponds to the first tracking pit Pw1 and the tracking pit Pw(2k) corresponds to the second tracking pit Pw2.
On the track 2k+1, a tracking pit Pw(2k+1) is provided opposite to the tracking pit Pw(2k) of the track 2k about a track center of the track 2k+1. Only the synchronizing pit P.sub.SYNC is formed on the track center. The tracking pit Pw(2k+1) corresponds to the first tracking pit Pw1 and the tracking pit Pw(2k) corresponds to the second tracking pit Pw2.
It is noted that the tracking pit Pw(2k) is commonly used in between the tracks 2k and 2k+1. Namely, the first and second tracks are formed in one track pitch Tp. Thus, the track pitch Tp is reduced to the half, 2L, and the number of tracks is increased twice to increase the recording density twice.
In order to reproduce such a disc without causing crosstalk, there has been proposed a super resolution reproducing system using a magnetically induced super resolution (MSR).
In the field of optics, it is known that the resolution of a microscope can be improved by providing an optical mask such as a pinhole on an object to be observed. The MSR improves the resolution not by providing a physical mask on surface of the magneto-optical disc, but by forming a mask, in effect, using a difference of temperatures of the medium, hence substantially increasing the spatial frequency. As a result, 1.5 to 3 times as much recording density can be obtained. For details, refer to the journal of Nippon Ouyou Jiki Gakkai, Vol. 15, Nov. 5, 1991.
Hence it is necessary to accurately control the distribution of temperature on the disc. Without such a control, the crosstalk between two adjacent recorded tracks may occur. The crosstalk renders it impossible to read information from the disc.